Optical filter opacity control for reducing temporal aliasing in motion picture capture

ABSTRACT

The present invention comprises a system for and method of frequency prefiltering comprising a camera shutter capable of continuously variable illumination during a single exposure of the sensor. The shutter comprises a continuously variable exposure effector which in disposed in an image path, either in front of a lens or between a lens and a sensor. The system for frequency prefiltering further comprises a synchronization cable that synchronizes a drive system with a sensor or with film. The shutter further comprises a postfilter. The postfilter comprises a digital finite impulse response convolutional filter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/381,341 filed Sep. 9, 2010 and the entire content is incorporated byreference herein and made part of this specification.

BACKGROUND OF THE INVENTION Field of the Invention (Technical Field)

The present invention relates to a shutter apparatus comprising acontinuously variable exposure effector and method for improvingfiltering in conventional two-dimensional (2D) or three-dimensional (3D)cameras resulting in reducing or preventing temporal aliasing.

Motion picture film cameras known by those having ordinary skill in theart expose film by allowing light to pass through a lens opening andthen through a shutter aperture. The shutter typically rotates at aspeed synchronous with that of the passing film frames. Digital motionpicture cameras expose the sensor by electronically controlling theexposure time for each frame to achieve the same effect.

The shutter in most motion-picture film cameras is a focal plane typeand is called a rotary disk shutter. Inside a film camera is a rotaryshutter shaped like a semicircle. FIG. 1 illustrates a typical motionpicture camera shutter consisting of an opaque region and an openregion. When the camera operates, the shutter mechanism turns to theopen position to let light hit the film and then continues turning,blocking light as the next frame moves into place.

A rotary disk shutter is a pie-shaped mirror disk that has a segment cutout, causing the shutter to have a light and dark cycle as the diskrotates. The film is exposed when the cutout segment of the shutterpasses in front of the film. The film is subsequently advanced duringthe dark cycle. As the mirror disk spins it reflects an image through aground glass so that it can be viewed by the camera operator part of thetime. The other part of the time the mirror allows the light to pass onto the film. On simple cameras this shutter is fixed and usuallysemi-circular. On more advanced cameras the shape of the shutter can beadjusted to a particular setting. This shutter setting is referred to asthe shutter angle.

Many motion-picture film camera shutters are adjustable. Imagine twosemicircles pinned together: the amount of space left open could neverbe more than 180 degrees (half a circle), but it could be less, if thetwo semicircles were fanned out so a smaller space is exposed. The angleof exposed space is called the shutter angle. The standard shutter anglein a film camera is 180 degrees. Sometimes angles of less than 180 areused for aesthetic or logistical reasons, and thus the amount of timeeach frame is exposed to light is actually less than half the framerate. Digital motion picture cameras do not have a mechanical shutter,so the resulting exposure time on digital cameras is controlledelectronically to simulate the same shutter angle controls.

Adjusting the shutter angle controls the proportion of time that thefilm is exposed to light during each frame interval. The primary reasonthat cinematographers adjust the shutter angle is to control the amountof motion blur that is recorded on each successive frame of film. Atight shutter angle will constrict motion blur. A wide shutter anglewill allow it. A 180° shutter angle is considered normal.

The use of a rotary disk shutter introduces aliasing. Aliasing literallymeans “by a different name” and is used to explain the effect ofunder-sampling a continuous signal, which causes real world frequenciesto be rendered in a sampled system as different frequencies. Thisaliased signal is the original signal shifted to a different frequencyand is usually seen as higher frequencies being aliased to lowerfrequencies.

Aliasing occurs when something continuous is represented by using lotsof discrete chunks, for example, representing pictures by using manypixels, storing sounds by taking many samples, or showing movement byusing many still pictures. The process of trying to represent continuousthings in discrete chunks inevitably distorts the information. Thisdistortion introduces ambiguities into the sound or pictures and “wrong”things are seen, i.e. aliases are seen. For example, for aone-dimensional audio signal in time, the aliased frequency componentssound lower in pitch. In a two-dimensional space, such as with images,parallel lines in pinstripe shirts aliasing into large wavy lines areobserved. For two-dimensional signals that vary in time, an example ofaliasing would be viewing propellers on a plane that seem to be turningslowly when the propellers are actually moving at very high speeds.

One way to describe a motion picture camera is as a sampling system intime. Motion picture cameras acquire images sequentially in time, witheach image representing a sample of the real world in time. In bothdigital and film based motion picture cameras, the time varying signalis measured at a fixed frame rate, usually 24 frames per second (fps).The sampling rate of typical motion picture cameras is usually 24 cyclesper second (or 24 hertz), so the sampling rate (in hertz) is the samenumber as the frame rate (in frames per second). This type of system maybe considered a time-sampling system. The performance of such a samplingsystem is analyzed and predicted with the well-known Nyquist-Shannonsampling theorem, which states: If a function x(t) contains nofrequencies higher than B hertz, it is completely determined by givingits ordinates at a series of points spaced 1/(2B) seconds apart.

The Nyquist frequency is defined as half the sampling frequency. Forexample, in a 24 frame per second (or 24 cycles per second, or 24 hertz)motion picture camera, the Nyquist frequency would be 12 hertz. TheNyquist-Shannon theorem predicts aliasing when real-world signals withfrequencies above the Nyquist frequency are sampled, i.e. any real worldsignal frequency above the Nyquist rate will be aliased, or shifted intoanother (false) frequency that can be represented by the samplingsystem.

Aliasing can and does occur when the real-world frequencies exceed theNyquist rate since motion picture cameras are sampled systems. Motionpicture cameras measure in three dimensions: two spatial dimensions (thetwo-dimensional image produced for every frame) and also time. Samplingis a discrete observation or measurement, while aliasing is an illusion,an appearance of something that is not, due to shortcomings of sampling.

To understand the time-domain sampling of a motion picture camera,consider a simple light source such as a light bulb photographed with amotion picture camera. If the intensity of the light bulb is modulatedsinusoidally, the intensity recorded by the film or digital sensorshould correspondingly represent samples of the time-varying brightnessof the light bulb, and upon playback the light intensity varying overtime should match the sine wave of the original light bulb. The realworld continuously varying intensity of the light bulb is recorded as afinite string of discrete values, one value for every frame of themovie.

In the sinusoidally varying light bulb example previously described,with a frequency of the sine wave of 10 hertz, with the light sampledwith a normal 24 frame per second camera system, the 10 hertz signal isaccurately recorded and reproduced because it is less than the Nyquistfrequency of 12 hertz. However, if the light bulb is varied sinusoidallyat 14 hertz, the recorded and reproduced frequency from a 24 frame persecond camera results is 10 hertz. This is because 14 hertz is 2 hertzabove the Nyquist frequency, so the resulting frequency is 2 hertz belowthe Nyquist frequency. This is an example of signal aliasing when afrequency higher than the Nyquist frequency is sampled.

Temporal aliasing in motion picture cameras is exhibited in other ways.The most common and popularly understood manifestation of temporalaliasing is known as the “wagon wheel” effect resulting from a rotatingwheel observed on a television or cinema screen. The effect arisesbecause wheels on film or video sometimes seem to rotate in a directionopposite to the direction of wagon movement, or at a speed that looks“wrong.” This effect is particularly pronounced when looking at anold-fashioned wheel on a carriage or wagon, because the greater thenumber of spokes a wheel has, the easier it is for this phenomenon to beobserved. Thus, a rapidly moving wagon wheel captured by a motionpicture camera appears to stop, reverse direction, or move slowly,depending on the rate of rotation. The higher frequencies or cycles ofthe rotating motion are aliased, or falsely shifted, to appear asdifferent frequencies. This temporal aliasing results from the limitedframe rate.

The above described backwards motion of wheels is illustrated in FIG. 2where wheel motion is illustrated as a series of still images or‘frames.’ FIG. 2A illustrates three such frames where the wheel isrotated by 7.5 degrees in each frame. FIG. 2B illustrates frames wherethe wheel is rotated by 42 degrees each time, e.g. the wheel is rotatedmore quickly than in FIG. 2A. When attention is focused on the smallwhite dot on the rim of the wheel the rotation is still clockwise, andin fairly large increments. But when attention is focused on the spokesof the wheel, the wheel appears to rotate anticlockwise in very smallsteps. If not for the white marker dot, a clockwise rotation of 42degrees would look identical to an anti-clockwise rotation of 3 degrees,due to the rotational symmetry of the wheel spokes.

The sequence of images that represent a wheel rotating quickly in onedirection happens to look identical to the sequence of images for thesame wheel rotating slowly in the opposite direction: this is an exampleof aliasing. There are many such aliases that can be created. If therotation happens to occur at exactly the right speed (45 degrees perframe for this particular example) the wheel will appear to standperfectly still. If it's very slightly faster, the wheel will appear torotate in the correct direction, but far too slowly with respect to theactual speed of the wheel rotation.

Aliasing decreases the quality of the motion picture. Current practiceto address the aliasing issue in moving pictures includes using motionblur. Motion blur entails making each frame an average over the wholeinterval between one frame and the next instead of making each image inthe sequence a sharp snapshot of where items are at a given instant intime. In the wagon wheel example previously described, with motion blur,each spoke is represented as a gray “blurred bar” through the 7.5 degreeangle that the spoke sweeps across in that one frame. The first imageshows each of the spokes slightly thicker and a bit blurred at theedges.

FIG. 2B is an illustration of wagon wheel spokes sweeping across 42degrees from one frame to the next, almost the same size as the intervalbetween the spokes. A gray blur rather than individual spokes results.Blurring eliminates aliasing because the wheel no longer appears to bemoving backwards, it just looks blurred, which is what is observed “inreal life” when looking at a fast-moving wheel.

Using motion blur to eliminate aliasing is not a straightforwardprocess. The method employed to eliminate aliasing in sampling systemsis to band-limit the real-world signal before the sampling takes place,thus ensuring that no frequencies above the Nyquist frequency areallowed to enter the sampling system. This is known as prefiltering andis usually accomplished with a low-pass frequency filter. The ideallow-pass frequency filter for prefiltering is unity (signal unaffected)below the Nyquist frequency, and zero (no signal allowed) above theNyquist frequency.

State of the art motion picture camera prefiltering using an exposurewindow and shutter angle has limitations. Motion picture cameras embodysome inherent prefiltering, because the amount of time the shutter isopen causes some motion blurring on a single frame/sample.

Exposure time for a frame is typically indicated as a shutter angle. A360-degree shutter angle indicates the frame is exposed for the entiretime of the sample, while a 180 degree shutter angle indicates the frameis exposed for half of the time between samples. For example, in a 24frame per second motion picture system, a 180-degree shutter wouldexpose each frame for 1/48 of a second, while a 360-degree shutter wouldexpose each frame for 1/24 of a second.

When the amount of light allowed to pass to the sensor, whether film ora digital sensor, during the frame time is plotted as a function oftime, the resulting plot describes how the incoming image intensitychanges over time. This change in intensity over time is called theexposure window function, or simply the window function. Exposure windowfunctions for motion picture shutters have a sharp transition between 0(no light) and 1 (full exposure). Existing motion picture cameras do notimplement values other than 0 and 1 because the shutter is either openor closed.

Filters are represented by response to a given frequency; one suchrepresentation is called the modulation transfer function, or MTF. Themodulation transfer function when expressed linearly is normalizedbetween 0 and 1, where 1 is full response to a given frequency and 0 isno response. There is a direct mathematical relationship between theexposure window function and the frequency prefilter. If an exposurewindow function is known, the resulting modulation transfer function ofthe prefilter can be calculated. FIG. 3 illustrates the MTFs of theeffective prefiltering of a 180-degree and a 360-degree shutter anglecompared with an ideal prefilter for a 24 frame per second system(Nyquist frequency is therefore 12 hertz).

Regardless of at what speed the film itself is running through thecamera, half the time the shutter is open, exposing the film, and halfthe time, the shutter is closed, advancing the film. The shutter speedof a film camera, or how long each image is exposed to light, is halfthe frame rate or how many images are exposed each second. The cinematicstandard frame rate of 24 frames per second uses a shutter speed half ofthat, i.e. 1/48 of a second.

Ideally, all frequencies above Nyquist would be eliminated beforesampling takes place, and all frequencies below Nyquist would bepreserved without attenuation. This ideal modulation transfer functionof a prefilter is plotted in FIG. 4.

If the illumination of the sensor is plotted over the course of anexposure, the resulting plot is called the exposure window function. Fora standard 180-degree shutter, the exposure window function has only twovalues: 0 (fully dark) and 1 (fully light). The function starts at 0(closed), and then instantly transitions to 1 (fully open and light). Itremains fully open at 1 for 1/48 seconds. It then instantly transitionsto 0. In the more general case, a plurality of exposure window functionsmight be considered, with values ranging infinitely between 0 and 1.

For 24 fps film, a 180° shutter is currently used so the film is exposedfor 1/48^(th) seconds. FIG. 5 illustrates a resulting illuminationwindow function of the average illumination of a sensor, where 0 is noillumination and 1 is full illumination, as a function of angle tracedout over a circle. Thus, the transition from open to shut is abrupt andthe sides of the so-called “boxcar” window are steep. When this shutteris rotated, the image illumination transmitted to the sensor is at zerointensity half of the time when the opaque region is in front of thesensor, and at full intensity the other half of the time when the openregion is in front of the sensor.

FIG. 6 illustrates a plot of exposure vs. time of an ideal exposurewindow function plotted over that of a window created by a semi-circularshutter. The ideal window is infinite in length, and therefore notphysically realizable.

Currently, general window functions exist in the known art, but windowfunctions applied to exposure having values other than 0 or 1 (fullyclosed or fully open) have not been applied to imaging systems. In orderto tune a good frequency response, data values need to be captured overan extended period of time and with exposure window functions that canproduce illumination other than fully dark and fully illuminated; i.e.continually varying illumination in between dark and light. The presentinvention addresses this deficiency by comprising an apparatus andmethod of producing continually varying illumination.

The present invention comprises a shutter apparatus and method of usingthe shutter that more closely approaches creating an ideal exposurewindow function, thus eliminating the abrupt open-close transition seenin cameras currently used. The shutter apparatus of the presentinvention thus reduces or eliminates aliasing.

The present invention comprises an improved analog filter and method ofusing the filter in time domain sampling. The present inventionadditionally comprises a method of implementing an analog filter on thetime domain sampling.

Objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention.

SUMMARY OF THE INVENTION

The present invention comprises a system for frequency prefilteringcomprising a sensor, a shutter comprising a continuously variableexposure effector, an electronic drive system comprising a motor drivefor generating an electronic control signal for driving the variableexposure effector, and a synchronization cable for synchronizing theelectronic signal generator and the electronic drive system with thesensor. The shutter further comprises a postfilter comprising a digitalfinite impulse response convolutional filter.

The variable exposure effector comprises a liquid crystal comprising aplate comprising a variable opacity panel or alternately comprising aplurality of panels disposed adjacent to a lens, or a rotatable exposurecontrol wafer. The rotatable exposure control wafer of the presentinvention comprises perforations disposed in a randomly distributedpattern or micropattern extending through the wafer. The rotatableexposure control wafer of the present invention also comprises rigidvariable color-neutral opacity material comprising a variablecolor-neutral opacity material comprising a transparent area, an opaquearea, and a semi-transparent area.

The system for frequency prefiltering further comprises at least oneadditional synchronization cable connecting the sensor to the electronicsignal generator, and an image splitter, and a camera phase shiftdetection system.

The camera phase shift detection system comprises a sequence wandcomprising a series of indicator devices, a control box, a camera, and asynchronization cable connecting the control box and the sequence wandcomprising a LED wand. The indicator devices comprise visibleelectromechanical or electromechanical indicator devices.

The variable exposure effector further comprises a first opticalpolarizer and a second optical polarizer wherein the first opticalpolarizer comprises a rotatable optical polarizer and the second opticalpolarizer of claim 17 comprising a rotatable optical polarizer.

The system for frequency prefiltering further comprises a lens, at leasttwo cameras viewing a scene through the lens, an image splitter adjacentto the lens; and an additional system for frequency prefiltering.

The present invention further comprises a method of frequencyprefiltering comprising creating an exposure window function, providingcontinuous exposure values, tuning a frequency response, sampling animage sequence, and reducing aliasing in the resultant sequence. Themethod of frequency prefiltering further comprises disposing a shutterin an image path, generating an electronic signal, driving the signal toan electronic image sensor, continuously varying illumination whileexposing the sensor, driving a variable exposure effector, andsynchronizing the electronic signal generator and the electronic drivesystem with the sensor.

The method of frequency prefiltering further comprises exposing a singleframe, directly modulating an electronic shutter of the sensor, andmodulating sensitivity.

The method of continuously varying illumination further compriseselectrically driving a liquid crystal, varying opacity of the liquidcrystal, disposing a plurality of liquid crystal panels adjacent to alens, and electrically rotating an exposure control wafer.

The method of continuously varying illumination further comprisesdisposing locally random perforations through the exposure controlwafer, rotating the exposure control wafer; and creating a globalaverage desired exposure window function, and rotating a rigid materialcomprising a variable color-neutral opacity exposure control wafer. Themethod of continuously varying illumination further comprises rotating afirst optical polarizer, adjusting the angular rotation rate of thefirst optical polarizer, producing an exposure window function, anddisposing a second optical polarizer adjacent to a first opticalpolarizer. The method of continuously varying illumination of claim 31further comprises rotating the second optical polarizer averaging overall polarization angles in the incident light and acquiring frames, andapplying a digital finite impulse response postfilter to the imagesequence, compensating for low-frequency loss resulting from theprefilter, and correcting the base band frequency to near unityresponse. The method of continually varying illumination is accomplishedusing a still photography camera or a time-lapse system.

The method of frequency prefiltering further comprises connecting thesensor to the electronic signal generator via at least one additionalsynchronization cable, disposing an image splitter, operating twosensors simultaneously, operating two exposure control effectorssimultaneously, operating two sensors and exposure control effectors 180degrees out of phase with one another; and interleaving the imagesequences from the two sensors to create a single resulting sequencewith desired frequency response.

The present invention further comprises a method of detecting cameraphase shift comprising indicating visible energy via a sequence wandcomprising a LED wand, controlling the sequence wand via a control box,disposing a camera for viewing the sequence wand, and connecting asynchronization cable to the control box and the sequence wand.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings in the attachment, which are incorporated intoand form a part of the specification, illustrate one or more embodimentsof the present invention and, together with the description, serve toexplain the principles of the invention. The drawings are only for thepurpose of illustrating one or more preferred embodiments of theinvention and are not to be construed as limiting the invention. In thedrawings:

FIG. 1 is an illustration of a typical motion picture camera shutterdisk;

FIGS. 2A and 2B illustrate an example of temporal aliasing;

FIG. 3 is an illustration of modulation transfer functions for typicalmotion picture cameras;

FIG. 4 is an illustration of an ideal prefilter modulation transferfunction;

FIG. 5 is a plot of an exposure window function;

FIG. 6 is a plot of exposure vs. time of an ideal exposure windowfunction and that of an exposure window function created by asemi-circular shutter;

FIG. 7A is a plot of the exposure window function over a single frame;

FIG. 7B is a plot of the modulation transfer function for an exposurewindow function;

FIG. 8 is a plot of the modulation transfer function for an exposurewindow function with postfiltering and prefiltering;

FIG. 9 is an illustration of a shutter comprising a continuouslyvariable exposure effector comprising a perforated wafer as a method ofproducing variable exposure window functions of the present invention;

FIG. 10 is an illustration of twin polarizers, one stationary and onerotating, as another method of producing variable exposure windowfunctions of the present invention;

FIG. 11 is a transparency function for design and fabrication ofexposure control wafers such as illustrated in FIGS. 9 and 13;

FIG. 12 is an illustration of twin polarizers, both rotating, as anothermethod of producing variable exposure window functions of the presentinvention;

FIG. 13 is an illustration of a shutter comprising a continuouslyvariable exposure effector comprising a graded neutral density wafer asa method of producing variable exposure window functions of the presentinvention;

FIG. 14 is an illustration of an image sequence recording system with arotating wafer capable of producing a variable exposure window functionof the present invention attached.

FIG. 15 is an illustration of an alternate embodiment of an imagingsystem with a rotating wafer capable of producing a variable exposurewindow function of the present invention attached;

FIGS. 16A and 16B illustrate an alternate embodiment of an imagingsystem comprising a liquid crystal display (LCD) shutter to producevariable exposure window functions;

FIG. 17 is an illustration of the camera of the present inventioncomprising a digital sensor controlled to produce variable exposurewindow functions;

FIG. 18 is an illustration of a two-camera system comprising a temporalaliasing reduction system employing wafers that produce variableexposure window functions;

FIG. 19 is an illustration of a two-camera system for temporal aliasingreduction comprising liquid crystal panels that produce variableexposure window functions;

FIG. 20 is an illustration of a two-camera system for temporal aliasingreduction employing direct exposure sensitivity control of electronicimage sensors such that variable exposure window functions are produced;

FIG. 21 illustrates a system for testing the temporal signal response ofany camera;

FIG. 22 illustrates an experimentally measured plot of modulationtransfer function of prior-art imaging systems;

FIG. 23 illustrates an experimentally measured modulation transferfunction plot for an imaging system employing the new exposure windowfunction of this invention;

FIG. 24 illustrates an experimentally measured modulation transferfunction plot for an imaging system employing the new exposure windowfunction of this invention as well as the postfiltering of thisinvention;

FIG. 25 illustrates a camera phase shift detection system;

FIG. 26 shows plots of a light bar out of calibration and one incalibration;

FIG. 27 illustrates a liquid crystal display (LCD) shutter;

FIG. 28 illustrates an example of exposure timing and LCD exposurecontrol; and

FIG. 29 is a plot illustrating exposure timing and LCD exposure control.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method and apparatus for reducingtemporal aliasing in motion picture cameras and improving theperformance of cameras.

An embodiment of the present invention comprises a prefilterincorporating exposure window functions shaped differently from windowfunctions currently used. The prefilter of the present inventionexhibits transitional values other than 0 (fully closed) and 1 (fullyopen). The use of the prefilter of the present invention results in adifferent, improved modulation transfer function that is produced priorto sampling.

The prefilter of the present invention produces exposure windows thatare continuously varied. The variation results in an MTF with improvedcapability to reduce aliasing frequencies.

FIG. 7 illustrates an exposure window resulting from the all embodimentsof the present invention and the resulting MTF compared to the MTFresulting from a 180-degree shutter. FIG. 7A is a plot of the exposurewindow function over a single frame at a shutter speed of 24 fps. FIG.7B is a plot of the modulation transfer function for the exposure windowof all embodiments of the present invention illustrated in FIG. 7A. Theresulting MTF has substantially less response above the Nyquistfrequency, but a slightly reduced response below Nyquist when comparedwith the MTF of a typical 180 degree shutter, also shown in FIG. 7B.

Another embodiment of the present invention comprises a postfilteringapparatus that improves the system's response to frequencies below theNyquist frequency, also called the baseband response. It is desirable tohave the frequencies below Nyquist represented as fully as possible,ideally at a response of 1.0. The postfiltering apparatus adjustsreductions in the response in the region below the Nyquist frequency.

The postfiltering apparatus comprises a digital finite impulse response(FIR) convolutional filter. The FIR filter is a one-dimensional filterin the time dimension, so for every pixel of a particular frame, thevalue for that pixel is replaced by the weighted sum of values from thatsame pixel in the current frame as well as previous and subsequentframes. The number of frames used for the weighted sum is determined bythe order of the filter i.e. how many elements are in the kernel. When athree element kernel is used, with values of [−1,41,−1], for every pixelin a frame, the pixel value is multiplied by 41, then the values of thatpixel previous and subsequent frames are subtracted from that value.Finally, the value is divided by 39 (the sum of the three elements inthe kernel) to normalize the sum. Higher order (length) kernels areemployed to tune or obrtain different frequency responses.

FIG. 8 is a plot of the MTF for an exposure window produced usingpostfiltering and illustrates the combined MTF of a prefilter and apostfilter. The combined MTF provides a response closer to 1.0, thedesired response in this area at frequencies below the Nyquist frequencyand reduced response at frequencies above the Nyquist frequency,compared with the MTF of a typical 180-degree shutter.

Another embodiment of the invention comprises a novel improved shuttercomprising a wafer wherein perforations, preferably micro-perforations,are disposed in a locally random pattern and extending through thethickness of the wafer. The wafer comprises a shape including but notlimited to a disk, a rectangle, a circle, an oval, or any otherfunctional shape. The shutter comprises an exposure control apparatus,or an exposure window function generator. The shutter comprises acontinuously variable exposure effector. The shutter enables theillumination captured on film or by a sensor to be varied. FIG. 9illustrates shutter wafer 90 comprising a perforated micropattern (notshown to scale). The average density of perforations is adjustablydistributed along path 92 producing a plurality of average opacities,thus creating any desired exposure window, in order words, creating aglobal average desired exposure window function.

A desired exposure window is created by disposing rotatable shutterwafer 90 adjacent to a lens or alternately near a sensor in a digitalcamera. The shutter wafer is rotated. The rotation is synchronized withthe rate of frame acquisition of the camera. Wafer 90 continues torotate so that area 94 rotates toward the vector of illumination. Theillumination is blocked progressively less as the density ofperforations becomes progressively greater. When area 98 is disposeddirectly adjacent to the illumination, more light is able to passthrough to the sensor than is passed through at area 94. As shutterwafer 90 continues rotating and area 96 is rotated toward the vector ofillumination, the illumination is blocked even less. When area 96 isdisposed directly adjacent to the illumination, the perforations are sodense that the wafer is nearly transparent to illumination. Thus, a widevariety of exposure window functions over time are created by changingthe average density of perforations in the wafer.

Another embodiment of the present invention comprises an apparatuscomprising two optical polarizers that continuously control illuminationby rotating the polarizing filters, thus reducing or increasingillumination as desired. FIG. 10 illustrates system for frequencyprefiltering 100 comprising polarizer 101 and polarizer 103 wherein therelative position of said polarizers control the delivery ofillumination to sensor 108 as a function of time. Polarization directionof polarizer 101 is indicated by vector 102. Polarization direction ofpolarizer 103 is indicated by vector 104. Polarizer 101 is stationary,while polarizer 103 is rotated by electronic drive system 105 and drivemotor 106.

An alternate embodiment of system for frequency prefiltering 100comprises alternating the relative position of polarizers 101 and 103 byinterchanging the polarizers and by disposing polarizers 101 and 103closer to sensor 108 with no change to the effect on the system. Anotherconfiguration of system for frequency prefiltering 100 comprisesdisposing polarizers 101 and 103 between lens 110 and the film ordigital sensor in camera 108. Electronic control box 107 controlselectronic drive system 106 as desired. Synchronization cable 109synchronizes electronic drive system 106 with sensor 108.

FIG. 7A is an exposure window function with exposure changing as afunction of time over the duration of a single frame of exposure. Anexposure window function such as in FIG. 7A is created by the prefilterof the present invention embodied in FIG. 10. When polarizationdirection vector 104 moves and aligns with fixed vector 102 additionalillumination is allowed to reach sensor 108 and transparency isincreased. When polarization direction vectors 102 and 104 arepositioned perpendicular to each other, no illumination is allowed tostrike sensor 108 and transparency approaches zero. Electronic controlsystem 107 controls the rotation of polarizer 103 and provides a widerange of programmed exposure window functions by precisely controllingthe angular speed of polarizer 103 during each acquired frame of sensor108. This control allows exposure window functions such as the oneillustrated in FIG. 7A to be produced.

FIG. 11 is a transparency function used to design exposure control wafers such as those illustrated in FIGS. 9 and 13. A plurality of functionsis used to create a plurality of exposure control wafers as desired.

FIG. 12 illustrates system for frequency prefiltering 120 which isanother embodiment of the present invention similar to system forfrequency prefiltering 100 illustrated in FIG. 10. System for frequencyprefiltering 120 comprises rotatable optical polarizer 121 and rotatableoptical polarizer 125. Polarization direction of polarizer 121 isindicated by direction vector 122 and polarization direction ofpolarizer 125 is indicated by direction vector 126. Motor drive 128 andelectronic drive system 127 power rotatable polarizer 125. Polarizer 125rotates at least one half revolution during the exposure time of asingle frame onto sensor 250 integrating all possible polarizationangles of the incoming light onto the frame.

Polarizer 121 rotates at a rate different than the rotation rate ofpolarizer 125. Motor drive 124 and electronic drive system 123 powerrotatable polarizer 121. Electronic control system 129 controls therelative rates of rotation of polarizer 121 and 125. Polarizer 121accelerates and decelerates and produces any desired exposure windowfunction. The exposure to sensor 250 is 0.0 when the directions of thepolarization vectors 122 and 126 are perpendicular, and the exposure isat maximum when the directions of vectors 122 and 126 are parallel.Synchronization cable 251 controls synchronization of electronic controlsystem 129 with sensor 250.

Depending on the sensor, synchronization signals are either generated bythe electronic drive system to control the start of frame acquisition ofthe sensor or the sensor generates signals that indicate that frameacquisition has begun. The electronic control system receives thesesignals and properly synchronizes the motion with the sensoracquisition. The instantaneous relative angle of polarization directions122 and 126 determines the instantaneous illumination of sensor 250.Thus, electronic drive system 129 is programmed to drive polarizers 121and 125 to produce a plurality of exposure window functions, includingthe exposure window function illustrated in FIG. 7A.

FIG. 13 illustrates rotatable shutter wafer 130 of the presentinvention. Shutter 130 comprises a continuously variable exposureeffector. Wafer 130 comprises a rigid material including but not limitedto glass. Wafer 130 is comprised of a material comprising variablecolor-neutral opacity. One embodiment of wafer 130 comprises atransparent area located at or around area 132, an opaque area locatedat or around area 136, and a semi-transparent segment located at oraround area 134. A path traced on the surface of wafer 130 produces adesired exposure window resulting from the variable transparency ofwafer 130. One such path is indicated by circle 138, which provides thetransparency function plotted in FIG. 11.

FIG. 14 illustrates image sequence recording and sampling system 140comprising rotatable exposure control wafer 141. Rotatable exposurecontrol wafer 141 is disposed in front of lens 142. Exposure controlwafer 141 comprises a wafer including the wafers illustrated in FIGS. 9and 13 and the rotatable optical polarizers comprising wafersillustrated in FIGS. 10 and 12. Wafer 141 controls the exposure windowduring each frame exposure for either still or motion picture cameras.Incoming light passes through spinning wafer 141. The intensity of thelight entering into lens 142 and on to camera 143 is varied according tothe type of wafer used, as described previously. Electronic drive system144 controls motor drive 145, which powers rotatable exposure controlwafer 141. Synchronization cable 146 is attachably disposed betweencamera 143 and electronic drive system 144 and synchronizes the positionof wafer 141 with the camera exposure.

The rotation of wafer 141 over the course of the frame acquisition ofthe camera 143 changes the amount of light over time, and thereforeproduces a continuously varying exposure window function as desired,such as the one plotted in FIG. 7A.

FIG. 15 illustrates alternate embodiment imaging system 150. System 150comprises exposure control wafer 151 disposed adjacent to lens 152,between lens 152 and sensor 153. Incoming light passes through rotatingwafer 151. Exposure control wafer 151 comprises a wafer comprising thewafers illustrated in FIGS. 9 and 13 and the rotatable opticalpolarizers comprising wafers illustrated in FIGS. 10 and 12, andcontrols the exposure window during each frame exposure for a still ormotion picture camera. Exposure control wafer 151 variably changes theintensity of the light from lens 152 before the light enters sensor 153.Electronic drive system 154 controls motor drive 156 which powersrotatable exposure control wafer 151. Synchronization cable 155 isattachably disposed between camera 153 and electronic drive system 154and synchronizes wafer position with camera exposure.

The rotation of wafer 151 over the course of the frame acquisition ofsensor 153 changes the light over a period of time, and thereforeproduces any of a plurality of continuously varying exposure windowfunctions, such as the one plotted in FIG. 7A.

Another embodiment of the present invention comprises an apparatuscomprising a liquid crystal display (LCD) shutter, as illustrated inFIG. 16. The shutter comprising liquid crystal 161 comprises acontinuously variable exposure effector. FIG. 16A illustrates imagingsystem 160 comprising liquid crystal 161 comprising a plate for creatinga time-varying exposure window function. The LCD shutter continuouslycontrols the instantaneous illumination of the sensor over the course ofa single exposure. System 160 comprises liquid crystal 161 disposedadjacent to lens 162 on the opposite side of liquid crystal 161 from thedisposition of sensor 164. Sensor 164 comprises an optical sensorcomprising film or alternately a digital sensor.

Liquid crystal 161 varies in opacity depending on drive voltage. Thecontrolled liquid crystal 161 attenuates incoming light from the sceneby variable amounts depending on the drive of electronic drive system163. Synchronization cable 165 synchronizes electronic drive system 163with camera 164. Liquid crystal 161 adjusts the light intensity andcreates exposure window functions such as that plotted in FIG. 7A.

FIG. 16B illustrates alternate imaging system 260 comprising liquidcrystal 161 disposed between lens 162 and film or digital sensor 164.Synchronization cable 165 synchronizes electronic drive system 163 withsensor 164. Depending on the sensor, synchronization signals are eithergenerated by the electronic drive system to control the start of frameacquisition of the sensor, or the sensor generates signals that indicatethat frame acquisition has begun. The electronic drive system receivesthese signals and properly synchronizes the motion via the motor drivewith the sensor acquisition. In both of the previously describedembodiments, the variable opacity characteristic of liquid crystal 161controls the instantaneous illumination of the sensor. The electricallycontrolled opacity of liquid crystal 161 produces a wide variety ofexposure window functions, such as the one plotted in FIG. 7A.

FIG. 17 illustrates another embodiment comprising system 170 comprisingcamera 172, digital sensor 174 disposed inside camera 172, andelectronic drive system 176. The electronic shutter of image sensor 174is directly modulated to achieve a desired exposure window function.Digital sensor 174 comprises a CCD or CMOS sensor that senses incominglight. Electronic drive system 176 generates an electronic controlsignal. Sensor 174 senses incoming light that is sensed by theproportion to the control signal generated by electronic drive system176.

Digital sensor 174 comprises a CCD or CMOS sensor designed with directelectrical control of the power to the photodiodes comprising an“electronic shutter,” or “bias line control.” When the bias (or power)to each pixel's photodiode is removed, the pixel is no longer sensitiveto light. When power is applied, the pixel becomes sensitive to light.Electronic drive system 176 drives the bias line of the sensor rapidlyto approximate an analog change in gain of the pixel. When the biasline, during the course of a single frame of exposure, is rapidlyswitched on and off, and the ratio of on time to off time is varied overthe course of that exposure, an exposure window function of any desiredshape is produced during the single frame exposure time. The drive fromthe control system thus varies and modulates sensitivity in the sensor.FIG. 7A illustrates a possible generated exposure window functionresulting from system 170.

FIG. 18 illustrates two-camera system 180 comprising a temporal aliasingreduction system. Wafers 184 and 183 comprise any one of the waferembodiments illustrated in FIG. 9 or 13 or the rotatable opticalpolarizers illustrated in FIGS. 10 and 12. Wafers 184 and 183 compriserotating exposure control wafers and provide instantaneous exposure tosensors 185 and 186. Sensors 185 and 186 comprise optical sensors, notlimited to film or digital sensors. Incoming light enters lens 181.Image splitter 182 splits the image. A controller controls the rotationof exposure window function wafer 183 so the exposure window functiondescribed upon it, or set by the relative positions of polarizationangles, is 180 degrees out of phase with the function on exposure windowwafer 184. Synchronization cables 188 and 189 attach and synchronizemotion drive controller 187 to sensor systems 185 and 186. Controller187 drives wafers 183 and 184 via motor drives 270 and 271.

This method of frequency prefiltering comprises connecting the sensor tothe electronic signal generator via at least one additionalsynchronization cable, disposing an image splitter, operating twosensors and two exposure control effectors simultaneously, operating twosensors and exposure control effectors 180 degrees out of phase with oneanother; and interleaving the image sequences from the two sensors tocreate a single resulting sequence with desired frequency response.

The two-camera system achieves a longer effective exposure windowfunction than a single camera achieves. Each camera samples and acquiresimages and image sequences at half the rate that a single cameraacquires. The frames from the two cameras are combined to create finalcontinuous footage. Each camera runs at 12 frames per second, with aresult that system 180 as a whole achieves 24 frames per secondeffective capture. The frames from each sensor are alternately playedback to create the final footage. Alternatively, the cameras run athigher rates, and prior-art digital downsampling filters are employed toachieve improved effective modulation transfer functions.

FIG. 19 illustrates two-camera system 190 for reducing aliasingcomprising liquid crystals comprising panels 193 and 194. Liquid crystalpanels 193 and 194 are similar to liquid crystal comprising plate 161illustrated in FIG. 16. Liquid crystal panels 193 and 194 control thetime exposure window function to each camera 195 and 196 in a similarway as does the embodiment illustrated in FIG. 16. Incoming light enterslens 191 and the image is split with image splitter 192. Liquid crystalpanel 193 is controlled such that it has an exposure window function 180degrees out of phase with the window function being created by thecontrol of liquid crystal panel 194.

Synchronization cables 198 and 199 synchronize electronic drivecontroller 197 with both camera systems, and drive the liquid crystalpanels. Two-camera system 190 achieves a longer effective exposurewindow function than a single camera can, and each camera acquires andsamples images and image sequences at half the rate of a single camera.The frames from the two cameras are interleaved or combined to createthe final continuous footage. Each camera runs at 12 frames per secondto achieve 24 frames per second effective capture. The frames from eachcamera are alternated to create the final footage. Alternatively, thecameras run at higher rates, and more specialized digital filterfunctions are employed to achieve improved effective modulation transferfunctions. The present invention thus comprises two cameras looking atthe same image at the same time with alternating sampling andacquisition, and thus results in improved performance.

FIG. 20 illustrates two-camera system 200 employing direct exposuresensitivity control of electronic image sensors 203 and 204, similar tothose illustrated in FIG. 17. System 200 controls the time exposurewindow in a similar way as the embodiment illustrated in FIG. 17controls. Incoming light enters lens 201. Image splitter 202 splits theimage. An exposure window function 180 degrees out of phase with theexposure window function of sensor 204 drives the exposure sensitivityof sensor 203. Electronic drive system 207 controls and modulates theacquisition time and exposure sensitivity of each sensor viasynchronization cables 205 and 206.

The two-camera system achieves a longer effective exposure windowfunction than a single camera does. Each camera acquires and samplesimages and image sequences at half the rate of a single camera. Theframes from the two cameras are subsequently interleaved or combined tocreate final continuous footage. Each camera runs at 12 frames persecond to achieve 24 frames per second effective capture. The framesfrom each are alternated to create the final footage. Alternatively, thecameras run at higher rates, and prior-art digital downsampling filtersare employed to achieve improved effective modulation transferfunctions.

FIG. 21 illustrates system 210 for testing the temporal signal responseof any camera system with or without the embodiments listed herein.Control box 214 powers one or more light emitting diodes whichilluminate light emitting diode panel 212. Control box 214 isunsynchronized with camera 216 being tested. Test LED panel 212 isdisposed adjacent to camera 216 so that the panel fills the field ofview of the camera. Control box 214 drives the light emitting diodes andsinusoidally varies the average illumination of panel 212. Over thecourse of a test, control box 214 varies the frequency of the sinusoidallight output of panel 212 in steps with indication signals in betweeneach frequency. Analysis of the resulting data from the camera resultsin easily identifiable frequency steps.

For testing a typical camera for time domain frequency response, thefrequency of the sinusoidal illumination of panel 212 is varied in 1hertz increments from 1 hertz to 120 hertz, although higher frequenciesmay be produced. Starting at a frequency of 1 hertz, the LED panel isilluminated with a 1 hertz sinusoidal intensity for a duration ofseveral (typically five) seconds. Then the frequency is increased to 2hertz and held at that frequency for several seconds. This process iscontinued for all frequencies desired. The images and image sequencesfrom the camera recording the entire illumination sequence are analyzedto determine the recorded frequency with respect to the illuminatedfrequency on panel 212, and the response (recorded amplitude of thesinusoid with respect to actual amplitude of panel 212) of the camera toeach frequency is analyzed. To ensure linear response of the camera toillumination levels of 212, and to compensate for any nonlinearresponse, a linear sweep of amplitude of illumination of 212 is producedand recorded by the camera on film or digital storage.

A delay, or phase shift, between the externally or internally generatedsynchronization signal and the actual time of image sequencing andacquisition may exist in cameras currently used. This phase shift existsfor many reasons, including an internal camera controller delay or thetime delay caused by CMOS rolling shutters. The amount of this timeshift often differs from camera to identical model of camera.

The phase shift inherent in a camera, if any, must be known in order tosuccessfully synchronize a physical action occurring in the field ofview of a motion picture camera with the exact time that an image iscaptured. This is required to synchronize any of the embodiments of anexposure window function generator contained therein to a camera system.Phase differences between synchronization signals and camera acuqisitionwhich are not correctly compensated will cause incorrect alignment ofthe exposure window function generated and the exposure of the sensor orfilm.

Another embodiment of the present invention comprises an apparatus andmethod to detect and synchronize phase shifts. FIG. 25 illustratescamera phase shift detection system 250 comprising sequence wand 251,control box 252, and camera 253. Control box 252 receivessynchronization signal 253 via synchronization cable 254 and alsocontrols sequence wand 251 based on the same synchronization signal.Sequence wand 251 comprises a plurality of visible electromechanical orelectroluminescent indicators 255.

Additional lights 256 are always illuminated to indicate a centerline,or crosshair, to provide a goal position to the user during calibration.Plurality of visible electromechanical or electroluminescent indicators255 are operated in a sequential fashion and complete one cycle everyframe period.

One embodiment of the present invention comprises a plurality of visibleelectromechanical or electroluminescent indicators 255 comprising aplurality of light emitting diodes (LEDs). Control system 252sequentially illuminates only one LED at a time of the plurality ofLEDs, thus creating a scanning pattern from left to right (orvice-versa). The control illumination sequence commences at the instanta synchronization signal is received via synchronization cable 254.

Plurality of visible electromechanical or electroluminescent indicators255 is disposed at a location in view of the motion picture camera. Whenthe shutter time of the camera is set to a very narrow angle, that is, avery short exposure, only one of the LEDs will be illuminated during thetime of the camera exposure. The inherent phase shift of the motionpicture camera is determined by observing which of the LEDs isilluminated. When the camera sensor exposure start time andsynchronization signal are exactly calibrated, the central LED isdetected by the sensor during exposure. As the phase shift increases,other LEDs are visible and detected by the sensor. The correct phaseoffset is determined by the user adjusting the phase offset with controlbox 252 until the central LED is visible to and detected by the camera'ssensor.

There is no currently available method or system for quantifying thephase of a motion picture camera, even though many methods and systemsfor synchronizing the frequency or frame rate of motion picture camerascurrently exist. It is sometimes necessary to quantify the phase, or theinherent delay in the camera, from when a synchronization signal occursto when a frame is actually captured, when motion picture cameras areused.

The system of the present invention comprises an array of lights thatare turned on and off in a particular sequence during the course of asingle frame. The timing and synchronization of the lights is based onthe synchronization signal sent to and from the motion picture camera.The light array is held in the camera's field of view and the camera isconfigured to run with a very narrow shutter angle. When the cameracaptures an image, only some of the lights in the array will beilluminated. By knowing which lights are illuminated, the sequence inwhich the lights illuminate, and the period of the synchronizationsignal, the phase of the camera can be determined.

FIG. 26 shows two plots, one depicting a camera and synchronizationsystem where the phase offset between synchronization and cameraexposure is very large, and one showing a system with no phasedifference, that is, a system in which image sampling and acquisitionand the synchronization signal occur simultaneously. The camera phaseoffset is unknown at the start of calibration and, in this example,results in the LEDs near the right end of plurality of visibleelectromechanical or electroluminescent indicators 255 being the onlyones from which light is sensed during the image sampling andacquisition.

When the user uses control box 254 to shift the starting point, or phaseshift, of the LED sequence relative to the synchronization pulsereceived on synchronization cable 254, the small section of LEDs whichare illuminated during the short exposure of camera 253 appears to shiftin the image sampled and acquired by the camera. This is adjusted untilLEDs are illuminated at reference point 256. This calibration offsetvalue of the time shift entered into the control box is equal to theinherent phase shift in the motion picture camera.

Another embodiment of the present invention comprises a system forautomated variable neutral density filtering. A liquid crystal disposedin front of a camera lens is driven by an electronic control box with asymmetric square wave drive of variable amplitude and zero averagevoltage. The amplitude of the square wave causes the opacity of theliquid crystal to vary, with increasing amplitude causing increasingopacity. By adjusting the amplitude of the square wave voltage, theliquid crystal's opacity is precisely controlled to achieve a desiredreduction in light to facilitate proper exposure.

FIG. 27 illustrates liquid crystal display (LCD) shutter 271 disposed infront of camera lens 272. Electronic control box 273 drives shutter 271and precisely controls the liquid crystal's opacity to achieve desiredreduction of incoming light 275 from scene 274 to facilitate properexposure. Synchronization signals are accepted by control box 273 viasynchronization cable 276. Overall control of the time-lapse sequence isdirected by externally provided time-lapse controller 277, whichindicates electronically to controller 273, via standard signalprotocols, the desired exposure for a particular frame.

This opacity control is particularly beneficial in the field oftime-lapse photography, especially time-lapse photography where the timebetween exposures is variable during the sequence of exposures. Often inthis case, it is desirable to have the shutter exposure time be relatedto the time between exposures. For instance, if a 180-degree shutterangle is desired for the entire time-lapse sequence, then the exposuretime for each frame would be exactly half the time between exposures, soif a picture is to be taken every second, the exposure time wouldtherefore be one-half of a second.

In the case of a nonlinear time-lapse sequence, the time betweenexposures may be different during the course of the sequence. If theeffective shutter angle is to remain constant while the time betweenexposures is changing, the absolute shutter exposure time will becomelonger or shorter for a particular frame of the sequence, which willresult in overexposure or underexposure. Compensating with the lensaperture is undesirable, as it will shift the focus depth-of-field. Thepresent invention consistently exposes each frame of the time lapsesequence by adjusting the liquid crystal's opacity and thus achieves thedesired exposure for a given shutter speed and aperture.

The electronic control interface for the liquid crystal combines theactual drive electronics, a user interface for exposure control, and acomputer interface for control by an external time-lapse controlcomputer.

FIG. 28 illustrates an example of exposure timing and LCD exposurecontrol for a variable time lapse sequence of three frames where it isdesired to have a 180-degree shutter for all three frames. The firstframe covers a time period of one second, and the shutter is thereforeopen for 0.5 seconds. The LCD is set to have 100% transparency for frameone. Frame two covers two seconds, so the shutter will therefore be openfor one second to achieve the 180-degree shutter angle. To compensatefor this exposure, the LCD is set to 50% transparency. Frame threecovers three seconds, so the shutter will be open for 1.5 seconds, andthe LCD is therefore set to 33% transparency. In all three frames, theexposure to CCD or film is the same, and the 180-degree shutter look ismaintained.

Another embodiment of the present invention comprises an apparatus andmethod to control the liquid crystal during each exposure to createtime-variable, continuous exposure window values during the shutterexposure to tune the frequency response of the system.

FIG. 29 is a plot illustrating exposure timing and LCD exposure controlthat maintains exposure for each frame as well as provides for temporalanti-aliasing and reducing aliasing. For each frame, the shutter of thecamera is open for the entire duration of the frame. The LCD opacity isadjusted during the frame to achieve the exposure window functiondesired for temporal anti-aliasing and reducing aliasing. Additionally,the overall opacity of the LCD is adjusted to compensate for the frametime. In frame one, which lasts one second, the LCD opacity peaks at100%. In frame two, which is two seconds long, the LCD opacity is half,or only 50% at peak. In frame three, which is three seconds long, theLCD opacity is one third, or 33% at peak. All three frames have the samemotion frequency response tuned in and anti-aliasing properties, andwill have the same exposure to the film or CCD.

The invention is further illustrated by the following non-limitingexamples.

Example 1

A standard motion picture camera was tested with the system illustratedin FIG. 21. A Red One digital cinematography camera was used, and waspositioned to face the LED panel in a darkened room in accordance withthe test setup shown in FIG. 21. The camera shutter angle was set to 180degrees, and the frame rate was 24 frames per second. The panel wasinitially dark. The panel was ramped linearly in illumination to fullbrightness, with the illumination being recorded. The illumination rampwas used to linearize all data from this test.

The LED panel was subsequently electrically powered to produce lightintensity that varied sinusoidally as the panel was illuminated atincreasing frequencies from 1 to 120 Hertz. The output was captured bythe camera. A modulation transfer function plot was produced by plottingthe measured amplitude, or modulation, of each sine wave recorded by thecamera. The resulting plot is shown in FIG. 22. The solid linerepresents the measured data, and the dashed line represents thetheoretical response. The Nyquist frequency, 12 Hertz, is indicated onthe plot with a vertical dashed line. FIG. 22 is the measured modulationtransfer function in the time domain of the camera.

Example 2

A standard motion picture camera was tested for tuning or obtaining atime domain frequency response with system 160 for creating atime-varying exposure window value shown in FIG. 7A with test system 210illustrated in FIG. 21. A Red One digital cinematography camera wasused. An LCD shutter with timing control system was attached to thefront of the lens. The LCD prefiltering system produced the exposurewindow function illustrated in FIG. 7A. The camera faced the LED panelin a darkened room. The camera shutter angle was set to 360 degrees, andthe frame rate was 24 frames per second. The panel was initially dark.The panel was ramped linearly in illumination to full brightness, withthe illumination being recorded. The illumination ramp was used tolinearize all data from this test.

The LED panel was subsequently driven to produce light intensity varyingsinusoidally in a series of increasing frequencies from 1 to 120 Hertz.The output was captured by the camera. A modulation transfer functionplot was produced by plotting the measured amplitude, or modulation, ofeach sine wave recorded by the camera. The plot is shown in FIG. 23. Thesolid line represents the measured data, and the dashed line representsthe theoretical response. The Nyquist frequency, 12 Hertz, is indicatedwith a vertical dashed line. FIG. 23 is the measured modulation transferfunction in the time domain of the camera.

Example 3

A standard motion picture camera with the embodiment illustrated in FIG.16A was tested with the test system illustrated in FIG. 21. A digitalpostfilter and a Red One digital cinematography camera were used. An LCDshutter with a timing control system was attached to the front of thelens, producing the exposure window function shown in FIG. 7A. Thecamera was placed to face the LED panel in a darkened room. The camerashutter angle was set to 360 degrees and the frame rate employed was 24frames per second. The panel was initially dark. The panel was rampedlinearly in illumination to full brightness, with the illumination beingrecorded. The illumination ramp was used to linearize all data from thefollowing test.

The LED panel was then driven to produce light intensity varyingsinusoidally in a series of increasing frequencies from 1 to 120 Hertz.The output was captured by the camera. The resulting digital framesequence was then sharpened along the time domain by application of athree-element convolutional filter.

A modulation transfer function plot was produced by plotting themeasured amplitude, or modulation, of each sine wave recorded by thecamera. The plot is shown in FIG. 24. The solid line represents themeasured data, and the dashed line represents the theoretical response.The Nyquist frequency, 12 Hertz, is indicated with a vertical dashedline.

The preceding examples can be repeated with similar success bysubstituting generically or specifically described operating conditionsof this invention for those used in the preceding examples.

Although the invention has been described in detail with particularreference to these preferred embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverall such modifications and equivalents.

1-20. (canceled)
 21. A system comprising: an image sensor configured to generate a plurality of consecutive image frames; an optical filter having an opacity that is adjustable, the optical filter being disposed in an optical path of the image sensor and configured to vary an amount of light incident on the image sensor responsive to a control signal; and a controller configured to generate the control signal to vary the opacity during individual image frame periods of the plurality of consecutive image frames so that: the opacity decreases during a first part of the individual image frame periods and increases during a second part of the individual image frame periods, and object motion above a Nyquist frequency is filtered by the optical filter prior to detection by the image sensor.
 22. The system of claim 21, wherein the controller is configured to generate the control signal to vary the opacity during the individual image frame periods so that the decrease in the opacity during the first part of the individual image frame periods and the increase in the opacity during the second part of the individual image frame periods are substantially symmetric about a minimum opacity level of the opacity during the individual image frame periods.
 23. The system of claim 21, wherein the first part of the individual image frame periods and the second part of the individual image frame periods have the same duration.
 24. The system of claim 21, wherein the opacity is substantially the same across an entire filter surface of the optical filter at a first time during the individual image frame periods.
 25. The system of claim 21, wherein the controller is configured to generate the control signal to vary the opacity during the individual image frame periods so that a minimum opacity level of the opacity occurs mid-way during the individual image frame periods.
 26. The system of claim 21, wherein the controller is configured to generate the control signal to vary the opacity during the individual image frame periods so that the opacity transitions from a first opacity level to a 50% opacity level and from the 50% opacity level to a second opacity level, the first opacity level being below the 50% opacity level and the second opacity level being above the 50% opacity level.
 27. The system of claim 21, wherein the optical filter comprises a variable opacity liquid crystal.
 28. The system of claim 21, wherein an amplitude of the control signal varies over time, and a level of the opacity during the individual image frame periods corresponds to the amplitude during the individual image frame periods.
 29. The system of claim 21, wherein the control signal is a square wave.
 30. The system of claim 21, wherein the optical filter is disposed before the image sensor in the optical path and after a lens in the optical path so that the light passes through the lens and the optical filter before reaching the image sensor.
 31. A method comprising: generating, with a processor, a control signal to control an opacity of an optical filter disposed in an optical path of an image sensor; varying the opacity during individual image frame periods of a plurality of consecutive image frames responsive to the control signal so that: an amount of light incident on the image sensor varies during the individual image frame periods, the opacity decreases during a first part of the individual image frame periods and increases during a second part of the individual image frame periods, and the opacity varies during the individual image frame periods in a way that diminishes an impact of temporal aliasing that is capturable by the image sensor; and generating, with the image sensor, the plurality of consecutive image frames responsive to the light incident on the image sensor.
 32. The method of claim 31, wherein said varying comprises varying the opacity during the individual image frame periods so that the decrease in the opacity during the first part of the individual image frame periods and the increase in the opacity during the second part of the individual image frame periods are substantially symmetric about a minimum opacity level of the opacity during the individual image frame periods.
 33. The method of claim 31, wherein the first part of the individual image frame periods and the second part of the individual image frame periods have the same duration.
 34. The method of claim 31, wherein said varying comprises varying the opacity during the individual image frame periods so that the opacity is substantially the same across an entire filter surface of the optical filter at a first time during the individual image frame periods.
 35. The method of claim 31, wherein said varying comprises varying the opacity during the individual image frame periods so that a minimum opacity level of the opacity occurs mid-way during the individual image frame periods.
 36. The method of claim 31, wherein said varying comprises varying the opacity during the individual image frame periods so that the opacity transitions from a first opacity level to a 50% opacity level and from the 50% opacity level to a second opacity level, the first opacity level being below the 50% opacity level and the second opacity level being above the 50% opacity level.
 37. The method of claim 31, wherein the optical filter comprises a variable opacity liquid crystal.
 38. The method of claim 31, wherein an amplitude of the control signal varies over time, and said varying comprises varying the opacity during the individual image frame periods so that a level of the opacity during the individual image frame periods corresponds to the amplitude during the individual image frame periods.
 39. The method of claim 31, wherein the control signal is a square wave.
 40. The method of claim 31, further comprising receiving the light through a lens and then the optical filter prior to the light reaching the image sensor. 